Manufactured articles in silicon oxide and/or other mixed metallic oxides and process for their preparation in “final” or “almost final” dimensions

ABSTRACT

An optical article, which is a preform for an optical lens, which is isotropic, consisting essentially of silicon oxide or silicon oxide in combination with one or more oxides of elements belonging to Groups III to VI of the Periodic Table, 
     the article having a dimensional precision which has tolerance to surface roughness and profilometric accuracy required in the spectral interval of 200-800 nm of the electromagnetic spectrum, 
     the tolerance being between one-half and one-tenth wavelength corresponding to the range of about 0.350-0.02 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical components in silicon oxideand/or other mixed metallic oxides having dimensional precision whichhas surface roughness tolerance and profilometric accuracy within thespecifications described for visible and ultraviolet spectrum ranges.

The above manufactured articles have “final” or “almost final”dimensions as they are obtained by the isotropic dimensional reduction(miniaturization) of amorphous monolithic materials, called aerogels, inturn prepared by means of cold moulding techniques based on sol-gelprocesses.

The process for the preparation of the above objects involves theaccurate geometrical definition of the aerogel by:

the cold filling of a suitable mould with a liquid colloidal dispersion,called sol, formed from specific chemical precursors;

the polycondensation of the sol to obtain the respective gels therefrom(gelation);

the supercritical drying of the gels until aerogels are obtained withdimensions corresponding to the mould used;

the isotropic reduction (miniaturization) of the amorphous monolithicaerogels thus obtained, consisting of silicon oxide alone or in thepresence of one or more oxides of elements belonging to groups III to VIof the Periodical Table and exceptionally also other groups.

2. Description of the Background

It is known that optical materials, and in particular transparentoptical materials such as silica or molten quartz and optical glass,owing to their hardness and fragility, are difficult to process as thedirect hot moulding of these optical components and devices is generallynot possible for reasons of product quality.

The traditional method for producing these optical elements is based onthe reduction of an adequate preform to the end product by means ofslow, precise grinding operations.

Whereas some of these operations, such as reduction with both a flat andspherical surface, can be automized, others, such as asphericalfinishing, require complicated manual processes.

This operational difficulty results in a limited process flexibility onan industrial scale and unreasonably high costs to obtain qualityproducts such as optical components and devices based on the aboveaspherical optical system.

Owing to these technological limitations the optical industry has triedto solve the problem in various ways.

One of these is the moulding at high pressure and temperature ofaspherical lenses and other optical components, directly fromappropriate preforms of the optical material desired; with this method,which requires extremely sophisticated equipment such as a hothydrostatic press, high quality products are obtained but also at a highcost and the process consequently necessitates very substantialinvestments.

One way of reducing the costs is by the use of organic opticalmaterials, in particular plastics.

These materials can be melted and moulded with much more economicalprocesses and can also be very easily processed with machine tools.

Unfortunately the dimensional precision of the optical products obtainedgenerally by melting, is negatively influenced both by theinsufficiently controllable shrinking of the material during the coolingoperation and by the change of liquid-solid phase which causes adimensional distorsion and deterioration of the optical quality of themanufactured article.

Also with the use of mechanical processing with machine tools, theoptical products obtained from plastic materials do not have anacceptable quality as the material cannot be accurately processes owingto the fact that it is too soft.

In addition, the products which can be obtained with the above plasticmaterials, by hot moulding or mechanical processing, suffer from limitedchemical and dimensional stability and do not reach the durabilitystandards established for inorganic optical materials.

It is also known that optical components with definite dimensions can beobtained by suitably treating a gel deriving from the hydrolysis of asilicon alkoxide.

For example, U.S. Pat. No. 4,680,049 describes a method for thepreparation of optical glass based on silicon oxide which involves aninitial hydrolysis of a silicon alkoxide, the drying of the above geland a final thermal syntherization treatment until an optical glass withdefinite dimensions is obtained.

These “final” optical products however have a very significant deviationwith respect to the profile of the aerogel, as is amply illustrated inFIG. 1.

The two diagrams shown in the above figure represent the configurationof the upper surface of the aerogel (diagram A) and the correspondingsurface of the densified product (diagram B) respectively.

In the mould in which the gel is prepared the corresponding surface isrigorously flat: it can be seen how the flat surface of the mould passesto a convex surface in the aerogel to end up as a concave surface in thedensified product.

The distorsion of the manufactured article is herein quantified asfollows:${{distorsion}\quad {from}\quad {mould}\quad {to}\quad {aerogel}} = {{\frac{20\quad {\mu m}}{3000\quad {\mu m}} \times 100} = {0.67\%}}$${{distorsion}\quad {from}\quad {aerogel}\quad {to}\quad {glass}} = {{\frac{40\quad {\mu m}}{2000\quad {\mu m}} \times 100} = {2\%}}$

This process, which herein is simply indicated as “compensateddistorsion process”, is severely limited in its industrial applicationsas there are difficulties in programming specific geometries of theproduct.

In fact, as there is no biunivocal, continuous correspondence betweenthe geometry of the mould and that of the product, there is also norational control of the final dimensions of the product itself.

Another attempt at developing the processing technology of opticalmaterials has been made using machine tools with a very high precisionnumerical control, having a diamond point so as to be also able toprocess hard materials such as quartz and optical glass and withmovement on air bearings to minimize the vibrations of the tool point.

These machines have been successfully developed in the last ten yearsand reach precision in the profile control of about a tenth of amicrometer and, under favourable conditions, even higher precision inthe control of the surface roughness; they are consequently capable offinishing an item with so-called “optical” precision, which means aprecision which is suitable for optics limited within the infraredspectrum range.

On the other hand, the above machines are still not adequate forapplications in visible and ultraviolet spectrum ranges owing to themore severe specifications of surface roughness and profilometricaccuracy required by optical laws within these spectrum ranges.

In addition, this high precision processing, which although economicallyconvenient in special applications such as mirror finishing by laser incopper, aluminium or other materials typically used in infrared, is notgenerally economical for obtaining transparent optical components basedon silica or inorganic glass, for numerous reasons including thehardness and fragility of the materials.

It is known in fact that these machines can be well used in theprocessing of typical materials for applications in infrared; this isdue to their processability characteristics which are much higher thanoptical glass.

This creates great difficulties in the spectrum ranges, where glass isthe prevalent material for which the technology of the single rotatingdiamond point (S.P.T.D.M.) cannot be used because of its fragility.

As described in Italian patent application MI-92A02038 filed by theApplicant, these high precision machine tools are used on intermediatesto obtain perfectly and completely isotropic optical components anddevices in “final” or “almost final” dimensions; the aboveintermediates, as they have the property of isotropically shrinking, aremonolithic aerogels ideally amorphous of silica and/or other metallicoxides produced according to the technolgy described in U.S. Pat. No.5,297,814.

SUMMARY OF THE INVENTION

The Applicant has now found that gels prepared with the technology ofU.S. Pat. No. 5,207,814, in suitable moulds, in accordance with what isdescribed in Italian patent application MI92A02038 which can be referredto for any possible point of interest, can be linearly miniaturized intodensified products which maintain the proportions of the mould with aprecision greater than one part out of 10,000.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate the configuration of the upper surface of theaerogel, and the corresponding surface of the densified product,respectively, of U.S. Pat. No. 4,680,049.

FIG. 2 illustrates an example of a profilometric determination of thepresent invention showing that the aspherical profile of the aerogel iscomparable to the theonetical profile.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In particular, therefore, the invention relates to the preparation ofthe above products, according to a process which involves the accurategeometrical definition of the aerogel by:

the cold filling of a suitable mould with a liquid colloidal dispersion,called sol, formed from specific chemical precursors;

the polycondensation of the sol to obtain gels (gelation);

the supercritical drying of the gels until aerogels are obtained withdimensions corresponding to the mould used;

the isotropic reduction (miniaturization) of the amorphous monolithicaerogels thus obtained, consisting of silicon oxide alone or in thepresence of one or more oxides of elements belonging to the III° to VI°Group of the Periodical Table and exceptionally also other groups.

These cold moulding techniques are based on the use of specialspecifically prepared moulds.

These moulds, having much greater dimensions than the manufacturedarticle, have an internal volume which is defined as a “homothetic copy”of the “end”-product itself, which is characterized in terms ofprofilometric accuracy, surface roughness and scaling ratio with theproduct itself.

The product thus obtained has “almost final” dimensions i.e. it requiresonly an optical polishing with the conventional methods or, at the best,it has “final” dimensions i.e. it does not require any conventionaloptical processing.

The overall result of the present invention is therefore the economicalproduction of optical components and devices made of silica glass orother optical glass using a new cold moulding technique based onspecific sol-gel synthesis processes.

The present invention consequently relates to optical articles,components or devices, with “final” or “almost final” dimensions andcompletely isotropic, consisting of silicon oxide, either alone or inthe presence of one or more oxides belonging to groups III to VI of thePeriodic Table, and exceptionally also other groups, said opticalarticles, components or devices having dimensional precision which hastolerance to surface roughness and profilometric accuracy required forthe visible and ultraviolet spectrum ranges, characterized in that saidtolerance being between ½ and {fraction (1/10)} wave lengthcorresponding to the range 0.350-0.02 micrometers and, preferably, equalto ¼ wave length corresponding on an average, in the visible range, to0.275 micrometers.

The above and other operating details will be explained in the followingillustrative examples which however do not restrict the scope of thepresent invention.

EXAMPLE 1

Preparation of Preforms of Pure Silica

An example is given of the preparation of silica glass disks, with adiameter of 2.5 cm and height of 1.0 cm, as preforms for optical lenses.

For this purpose, 80 ml of HCl 0.01N are added, under vigorous stirring,to 100 ml (0.44 moles) of tetraethylorthosilicate (TEOS) (molar ratioTEOS:H₂O:HCl=1:10:1.8×10⁻⁴).

After about 60 minutes a limpid solution is obtained and 52.8 g ofcolloidal silica powder (Aerosil OX50-Degussa) prepared from silicontetrachloride by oxidation at high temperatures, is added, still undervigorous stirring, to this solution.

The mixture obtained is homogenized using ultrasounds for a duration ofabout ten minutes and then clarified by centrifugation.

The homogeneous dispersion obtained is poured into cylindricalcontainers of polyester with a diameter of 5.0 cm and height of 2.0 cm,which are hermetically closed, placed in an oven and maintained at 50°C. for 12 hours.

The gel which is obtained is suitably washed with ethanol andsubsequently supercritically dried in an autoclave at a temperature of300° C. or in any case higher than the critical temperature of thesolvent.

An aerogel is obtained which is calcinated at a temperature of 800° C.in an oxidizing atmosphere.

During the heating, the residual organic products coming from thetreatment in the autoclave are burnt.

The dimensions of the aerogel obtained are those of the internal volumeof the initial cylindrical container.

The disk of silica aerogel, after calcination, is subjected to a streamof helium containing 2% of chlorine, at a temperature of 800° C. and fora duration of 30 minutes to remove the silanolic groups present; theaerogel disk is finally heated in a helium atmosphere to a temperatureof 1400° C. for the duration of one hour so that the silica reachescomplete densification and consequent miniaturization.

After cooling, the disk reaches the desired final dimensions (diameter2.5 cm and height 1.0 cm), maintaining a homothetic ratio with the formof the initial aerogel determined by the initial mould.

The densified material has the same physicochemical characteristics asthe silica glass obtained by melting (density=2.20; refraction index (at587.56 nm)=1.4585; Abbe dispersion=67.6).

EXAMPLE 2

Duplication of Optical Surfaces

Moulds are prepared with an internal surface finished with opticalspecifications (surface roughness less than ⅕ with a wave lengthcorresponding to less than 0.08 micrometers).

The internal volume of the moulds corresponds to a cylinder of 5.0 cm indiameter and 2.0 cm in height.

One of the bases of the cylinder consists of the optical surface to beduplicated.

A colloidal solution prepared by adding to the homogeneous solution,obtained as in example 1, a solution of ammonium hydroxide 0.1N,dropwise under stirring, until a pH of about 4-5 is reached, is pouredinto the moulds.

The moulds thus filled, are hermetically closed, placed in an oven andmaintained at 20° C. for 12 hours.

The production of the gel and subsequent supercritical drying arecarried out according to the procedure described in example 1.

The profilometric and surface roughness results, measured on the opticalsurface of the aerogel, have the same optical quality as the originalsurface with a roughness of less than 0.1 micrometres, corresponding to⅕ average wave length of the visible spectrum range.

EXAMPLE 3

Aspherical Lenses

A mould has been designed for providing a preform for a flat/convex lensof which the convex surface corresponds to an aspherical surface definedby the general equation:$X = {\frac{{cy}^{2}}{1 + \sqrt{1 - {\left( {K + 1} \right)c^{2}y^{2}}}} + {Dy}^{4} + {Ey}^{6} + {Fy}^{8} + {Gy}^{10}}$

wherein the y axis of the equation corresponds to the optical axis ofthe lens.

The constants for the densified product, having a diameter of 15 mm±0.05and height of 6.25 mm±0.10, are the following:

C=0.17364596

K=−1.000000

D=−0.000071

E=0.000022

F=−6.62323E⁻⁷

G=7.03174E⁻⁹

To obtain the specific dimensions of the densified product, aminiaturization factor was programmed equal to 2, which is equivalent toan internal mould volume with double dimensions with respect to themanufactured article desired.

The transformations for the new constants are:

C′=C/R

K′=K

D′=D/R³

E′=E/R⁵

F′=F/R⁷

G′=G/R⁸

The appropriate mould was prepared with machine tools having numericalcontrol.

No optical finishing treatment was carried out on the surface of themould, the objective of the experiment being the average profile of theaspherical lens rather than the optical finishing of the surface.

A silicic sol was prepared with the procedure of example 2.

A series of 3 aerogels was prepared using the above mould according tothe procedures described in example 2.

The aerogels were subjected to profilometric analysis as follows: eachaerogel was placed in line at the centre of a Mitutoyo series 332profile projector and compared to the theoretical profile correspondingto the equation of the aspherical profile.

The comparison was carried out by direct placement over the screen.

To increase the sensitivity of the method, each analysis was carried outwith photographic aid and subsequent projection on a huge screenproviding a sensitivity of up to a ten thousandth of the dimension ofthe object.

The aerogels were then densified (miniaturized), with the thermaltreatment described in example 1 and compared with the respectivetheoretical profile as in the case of the aerogels.

In both the aerogels and the densified products, the maximum deviation,relating to the respective theoretical profiles is less than 0.002 mm, avalue which is considered as the limit of the sensitivity of the method.

An example of profilometric determination is shown in FIG. 2 wherein theaspherical profile of the aerogel is comparable to the theoreticalprofile generated by the equation (see the dark external line) and thesite of the theoretical profile points has been slightly moved towardsthe outside to facilitate observation of the trend parallel to thesurface.

In addition to the profilometric analysis, the dimensionalreproducibility was verified, by micrometry, on the main diameters (flatsurface) of the densified products.

The results are summarized in Table 1 below:

TABLE 1 SAMPLE AVERAGE DIAMETER (mm) STAND. DEVIATION A 34/44-1 15.37750.003 A 34/26 15.3725 0.002 A 34/28 15.3780 0.003

What is claimed is:
 1. An optical article, which is a preform for anoptical lens, which is an isotropically miniaturized, amorphousmonolithic aerogel, consisting essentially of silicon oxide or siliconoxide in combination with one or more oxides of elements belonging toGroups III to VI of the Periodic Table, said article having a surfacehaving a dimensional precision which has tolerance to surface roughnessand profilometric accuracy required in the spectral interval of 200-800nm of the electromagnetic spectrum, said tolerance being betweenone-half and one-tenth wavelength corresponding to the range of about0.350-0.02 μm.
 2. The optical article of claim 1, which has a toleranceequal to one-quarter wavelength corresponding, in the visible range toabout 0.275 μm.
 3. The optical article of claim 1 wherein the oxides ofthe elements belonging to Groups III to VI of the Periodic Table areselected from the group consisting of germanium oxide and titaniumoxide.
 4. The optical article of claim 1, wherein said isotropicallyminiaturized, amorphous monolithic aerogel is a densified andminiaturized silica aerogel.
 5. A manufactured article, which is anoptical lens, which is an isotropically miniaturized, amorphousmonolithic aerogel, consisting essentially of silicon oxide or siliconoxide in combination with one or more oxides of elements belonging toGroups III to VI of the Periodic Table, said article having a surfacehaving a dimensional precision which has tolerance to surface roughnessand profilometric accuracy required in the spectral interval of 200-800nm of the electromagnetic spectrum, said tolerance being betweenone-half and one-tenth wavelength corresponding to the range of about0.350-0.02 μm.
 6. The manufactured article of claim 5, which has atolerance equal to one-quarter wavelength corresponding, in the visiblerange, to about 0.275 μm.
 7. The manufactured article of claim 5,wherein the oxides of the elements belonging to Groups III to VI of thePeriodic Table are selected from the group consisting of germanium oxideand titanium oxide.
 8. The manufactured article of claim 5, wherein saidisotropically miniaturized, amorphous monolithic aerogel is a densifiedand miniaturized silica aerogel.